Layer orientation control for pixel-based additive manufacturing

ABSTRACT

A method is provided for making a workpiece in an additive manufacturing process in which a starting material is solidified in a layer by layer fashion, with each layer of the workpiece being solidified using a patterned image of radiant energy configured as a two-dimensional grid array of pixels. The method includes: for each layer of the workpiece, determining a preferred angular orientation of the grid array, relative to the layer; and orienting the patterned image to the preferred angular orientation before solidifying the starting material for that layer.

BACKGROUND OF THE INVENTION

This invention relates generally to additive manufacturing, and moreparticularly to apparatus and methods for process control in pixel-basedadditive manufacturing.

Additive manufacturing is a process in which material is built uplayer-by-layer to form a component. Stereolithography is a type ofadditive manufacturing process which employs a vat of liquid ultraviolet(“UV”) curable photopolymer “resin” and an image projector to buildcomponents one layer at a time. For each layer, the projector flashes alight image of the cross-section of the component on the surface of theliquid, or just above a transparent lens at the bottom of the resin. Theimage is formatted as a grid array of pixels. Exposure to theultraviolet light cures and solidifies the pattern in the resin andjoins it to the layer below or above, depending on the specific buildmethodology.

The pixels are inherently square or rectangular. The dimensions of thepixels are can be in the range of 20-100 μm, with 40-80 μm being morecommon.

One problem with this method is that, no matter how small the pixelsare, there will still be situations in which the edge of the area to becured is not in alignment with a pixel, and the pixel protrudes past theintended border. This type of error is referred to as “stair stepping”.

BRIEF DESCRIPTION OF THE INVENTION

This problem is addressed by a method of pixel-based additivemanufacturing in which an angular orientation of each build layer is setindependently.

According to one aspect of the technology described herein, a method isprovided for making a workpiece in an additive manufacturing process inwhich a starting material is solidified in a layer by layer fashion,with each layer of the workpiece being solidified using a patternedimage of radiant energy configured as a two-dimensional grid array ofpixels. The method includes: for each layer of the workpiece,determining a preferred angular orientation of the grid array, relativeto the layer; and orienting the patterned image to the preferred angularorientation before solidifying the starting material for that layer.

According to another aspect of the technology described herein, a methodis provided for making a workpiece. The method includes: placing anuncured resin in a vat; positioning a build platform in the resin at aselected location along a first axis, so as to expose a layer of resin;rotating at least one of the build platform and a projector about thefirst axis, so as to orient the projector at a predetermined angularorientation relative to the exposed layer of resin; using the projector,selectively curing the layer of resin by projecting a patterned image ofradiant energy onto the layer of resin, wherein the patterned image isconfigured as a grid pattern comprising rows and columns of pixelsarrayed along second and third mutually perpendicular axes respectively,wherein the second and third axes are perpendicular to the first axis,and wherein the grid pattern is aligned at the predetermined angularorientation.

According to another aspect of the technology described herein, anapparatus for making a workpiece includes: a vat configured to contain aliquid resin; a platform movable along a build axis within the vat; aprojector operable to project a patterned image of radiant energycomprising rows and columns of pixels; and means for changing a relativeangular orientation of the platform and the projector about the buildaxis.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawing figuresin which:

FIG. 1 is a schematic diagram illustrating a stereolithographyapparatus;

FIG. 2 is a schematic perspective view of an exemplary workpiece thatcan be constructed using the apparatus of FIG. 1;

FIG. 3 is a view taken along lines 3-3 of FIG. 2;

FIG. 4 is a view taken along lines 4-4 of FIG. 2;

FIG. 5 is a view of a layer of the workpiece shown in FIG. 2 with a gridpattern overlaid thereon;

FIG. 6 is a view of a layer of the workpiece shown in FIG. 2 with a gridpattern overlaid thereon in a nominal orientation; and

FIG. 7 is a view of a layer of the workpiece shown in FIG. 2 with a gridpattern overlaid thereon in an optimized orientation.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 illustratesschematically an stereolithography apparatus 10 suitable for carryingout an additive manufacturing method as described herein. Basiccomponents of the apparatus 10 include a vat 12 containing aphotopolymer resin 14, a platform 16 connected to an actuator 18, aprojector 20, and a controller 22. Each of these components will bedescribed in more detail below.

The platform 16 is a rigid structure defining a planar worksurface 24.For purposes of convenient description, the plane of the worksurface 24is oriented parallel to an X-Y plane of the apparatus 10, and adirection perpendicular to the X-Y plane is denoted as a Z-direction (X,Y, and Z being three mutually perpendicular directions). The Z-directionor Z-axis may also be referred to herein as a “build axis”.

The actuator 18 is operable to move the platform 16 parallel to theZ-direction. It is depicted schematically in FIG. 1, with theunderstanding devices such as pneumatic or hydraulic cylinders,ballscrew or linear electric actuators, and so forth, may be used forthis purpose.

The projector 20 may comprise any device operable to generate a radiantenergy patterned image of suitable energy level and other operatingcharacteristics to cure the resin 14 during the build process, describedin more detail below. In the illustrated example, the projector 20comprises a radiant energy source 26 such as a UV lamp, an image formingapparatus 28 operable to receive a source beam B from the radiant energysource 26 and generate a patterned image P comprising an array ofindividual pixels to be projected onto the surface of the resin 14, andoptionally focusing optics 30, such as one or more lenses.

The radiant energy source 26 may comprise any device operable togenerate a beam of suitable energy level to cure the resin 14. In theillustrated example, the radiant energy source 26 comprises a UV flashlamp.

The image forming apparatus 28 may include one or more mirrors, prisms,and/or lenses and provided with suitable actuators, and arranged so thatthe source beam “B” from the radiant energy source 26 can be transformedinto a pixelated image in an X-Y plane coincident with the worksurface24. In the illustrated example the image forming apparatus 28 may be adigital micromirror device.

The controller 22 is a generalized representation of the hardware andsoftware required to control the operation of the apparatus 10,including the projector 20 and actuator 18. The controller 22 may beembodied, for example, by software running on one or more processorsembodied in one or more devices such as a programmable logic controller(“PLC”) or a microcomputer. Such processors may be coupled to sensorsand operating components, for example, through wired or wirelessconnections. The same processor or processors may be used to retrieveand analyze sensor data, for statistical analysis, and for feedbackcontrol.

Generically, a build process begins by positioning the platform 16 justbelow the surface of the resin 14, thus defining a selected layerincrement. The projector 20 projects a patterned image P representativeof the cross-section of the workpiece on the surface of the resin 14.Exposure to the radiant energy cures and solidifies the pattern in theresin 14. The platform 16 is then moved vertically downward by the layerincrement. The projector 20 again projects a patterned image P. Exposureto the radiant energy cures and solidifies the pattern in the resin 14and yet joins it to the previously-cured layer below. This cycle ofmoving the 16 and then curing the resin 14 is repeated until the entireworkpiece is complete.

Additionally, means are provided for rotating the projector 20 and theplatform 16 relative to each other about the Z-axis. Rotation of eitherthe projector 20, or the platform 16, or both are suitable to carry outthe method described herein. In the illustrated example, an actuator 32is provided which is operable to controllably rotate the projector 20.

FIG. 2 shows an exemplary workpiece 34 which takes the form of anelongated hollow structure having opposed side walls 36 and opposed endwalls 38, extending between a lower end 40 and an upper end 42. FIG. 3illustrates a representative plan view of the workpiece 34 at the lowerend 40. The workpiece 34 is oriented such that the side walls 36 areparallel to a nominal X-axis direction (corresponding to the X-axis ofthe stereolithography apparatus 10 described above). The end walls 38are perpendicular to the side walls 36, and are therefore parallel to anominal Y-axis direction (corresponding to the Y-axis of thestereolithography apparatus 10). FIG. 4 illustrates a representativeplan view of the workpiece 34 at the upper end 42, with the workpiece inthe same orientation as FIG. 3. At this location it can be seen that theside walls 36 are not parallel to the nominal X-axis direction, and theend walls 38 are not parallel to the Y-axis. Stated another way, thecross-sections of the workpiece at the lower and upper ends 40, 42 arerotated relative to each other about a nominal Z-axis direction. Thistype of structure may be described as “twisted”.

In order to produce the workpiece 34 using the apparatus 10, theworkpiece 34 is modeled as a stack of planar layers arrayed along theZ-axis. It will be understood that the actual workpiece 34 may bemodeled and/or manufactured as a stack of dozens or hundreds of layers.

FIG. 5 illustrates a single representative layer at the lower end 40 ofthe workpiece 34. The cross-sectional shape of the workpiece 34 overlaidwith a grid array of pixels 44 comprising mutually perpendicular rows(X-direction) and columns (Y-direction). The pixels 44 may bequadrilateral shapes such as rectangles or more particularly squares.With the workpiece 34 in this particular orientation, each edge of theworkpiece 34 is parallel to either the X-axis or the Y-axis. Thus it isapparent that the edge of a rectangular pixel parallel to the X-axis orY-axis could always be aligned with the workpiece edge, given a suitablysmall pixel dimension.

In contrast, FIG. 6 illustrates a single representative layer taken atthe upper end 42 of the workpiece 34, again overlaid with a grid ofpixels 44. Because no edge of the workpiece 34 is parallel to either theX-axis or the Y-axis, it is not always possible to align the edge of arectangular pixel parallel to a workpiece edge. Accordingly, no matterhow small the pixel dimension, some degree of stair-stepping will occur,as described above.

To counter this effect and improve part fidelity, the angularorientation of the projector 20 (and thus the X-Y grid orientation)relative to the workpiece 34 may be individually selected for eachlayer.

For example, in the process of modeling and building the layer shown inFIG. 4, the layer orientation may be set at a nominal angularorientation, which is herein identified as being at 0° rotation. Asnoted above, flashing of the patterned image P may occur at the nominalangular orientation, with the expectation that the edges of pixels willbe substantially aligned with the edges of the workpiece 34.

In order to model and build a layer shown in FIG. 6, the layerorientation may be set at a different angular orientation in order toachieve the best correspondence between the X-Y grid orientation and theworkpiece edges. In the illustrated example, the workpiece cross-sectionat the upper end 42 is rotated approximately 30° counterclockwiserelative to the workpiece cross-section at the lower end 40.Accordingly, as shown in FIG. 7, the X-Y grid orientation may be rotatedapproximately 30° counterclockwise. During the build process, theprojector 20 would be rotated approximately 30° counterclockwise and theflashing of the patterned image P may occur at this new angularorientation, with the expectation that the edges of pixels 44 will againbe substantially aligned with the edges of the workpiece 34. This newoff-nominal orientation may be referred to as a “preferred orientation”or an “optimized orientation”.

For a simple workpiece involving a twisted prismatic shape as describedabove, or other type of component having edges which are orthogonal toeach other but rotated relative to the nominal X-Y axes, there is likelya specific angular orientation for each layer which provides exactcorrespondence between the pixel edges and the workpiece edges.

For other types of workpiece sectional shapes, it is possible to use asoftware optimization algorithm to determine an optimized fit or bestfit for each layer. For example, it is possible to compute for a givenangular orientation how many pixels 44 in the layer would cross a partedge, or to compute for a given angular orientation the total surfacearea of pixels 44 crossing a part edge. The orientation may then bevaried and the computations repeated until one or more of these valuesare minimized, resulting in an optimized angular orientation.

In any case, because the pixels 44 are individually addressable alongtwo mutually perpendicular axes, the optimum angular orientation foreach layer may be achieved with only a 90° range of rotation.

It is also possible when determining angular orientation of a layer toconsider the fidelity and surface quality of the workpiece along theZ-axis. For example, considering the twisted prismatic workpiece 34described above, it is apparent that if the optimum angular orientationis used for each layer individually, the exterior surface of thefinished workpiece 34 may exhibit a “terraced” effect. In order toreduce or minimize this effect, the angular orientation of each layermay be alternated between a nominal (0°) orientation and an optimizedorientation, or the orientation may be randomized for each layer. Thiswould produce improved surface quality along the Z-axis, at the expenseof incurring some amount of stair stepping in the individual layers.

The method described herein has several advantages over the prior art.It method enhances fidelity for pixel-based layer manufacturedcomponents. This process ensures that various pixels are utilized withrespect to the various geometries and their relative locations in thebuild to optimize the features at those respective locations in thecomponent. It will provide increased ability to produce critical partgeometry, reduce variability and increase yield, resulting in lower partcost.

It is noted that the stereolithography apparatus 10 described above ismerely an example used for the purposes of explanation. The methoddescribed herein is effective with any method of additive manufacturingin which a starting material is solidified, where the solidifying forceis applied using a grid of pixels.

The foregoing has described an apparatus and method for layerorientation control in a pixel-based additive manufacturing process. Allof the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings) may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying potential points of novelty, abstract and drawings), orto any novel one, or any novel combination, of the steps of any methodor process so disclosed.

What is claimed is:
 1. A method of making a workpiece in an additivemanufacturing process in which a starting material is solidified in alayer by layer fashion, with each layer of the workpiece beingsolidified using a patterned image of radiant energy configured as atwo-dimensional grid array of pixels, the method comprising: for eachlayer of the workpiece, determining a preferred angular orientation ofthe grid array, relative to the layer; and orienting the patterned imageto the preferred angular orientation before solidifying the startingmaterial for that layer.
 2. The method of claim 1 wherein each pixel hasa quadrilateral shape.
 3. The method of claim 2 wherein each pixel has arectangular shape.
 4. The method of claim 1 wherein the preferredangular orientation is selected to align an edge of one or more pixelswith an edge of the workpiece.
 5. The method of claim 1 wherein thestarting material is a photopolymer and the patterned image is projectedusing ultraviolet light.
 6. The method of claim 1 wherein the preferredangular orientation is selected randomly for two or more layers.
 7. Themethod of claim 1 wherein the preferred angular orientation alternatesbetween a nominal angular orientation and an off-nominal angularorientation.
 8. The method of claim 1 wherein the preferred angularorientation is determined by an optimization algorithm which minimizesthe number or area of pixels crossing an peripheral edge of theworkpiece.
 9. The method of claim 1 wherein the layers are stacked alonga build axis, and the angular orientation of the layers are selectedconsidering surface quality of the workpiece along the build axis.
 10. Amethod of making a workpiece, comprising: placing an uncured resin in avat; positioning a build platform in the resin at a selected locationalong a first axis, so as to expose a layer of resin; rotating at leastone of the build platform and a projector about the first axis, so as toorient the projector at a predetermined angular orientation relative tothe exposed layer of resin; and using the projector, selectively curingthe layer of resin by projecting a patterned image of radiant energyonto the layer of resin, wherein the patterned image is configured as agrid pattern comprising rows and columns of pixels arrayed along secondand third mutually perpendicular axes respectively, wherein the secondand third axes are perpendicular to the first axis, and wherein the gridpattern is aligned at the predetermined angular orientation.
 11. Themethod of claim 10 wherein each pixel has a quadrilateral shape.
 12. Themethod of claim 10 wherein the predetermined angular orientation isselected to align an edge of one or more of the pixels with an edge of aworkpiece.
 13. The method of claim 10 further comprising repeating in acycle the steps of positioning, rotating, and projecting to build up theworkpiece in a layer-by layer fashion.
 14. The method of claim 13wherein the predetermined angular orientation is selected randomly fortwo or more layers.
 15. The method of claim 13 wherein the predeterminedangular orientation alternates between a nominal angular orientation andan off-nominal angular orientation.
 16. The method of claim 13 whereinthe preferred angular orientation is determined by an optimizationalgorithm which minimizes the number or area of pixels crossing anperipheral edge of the workpiece.
 17. The method of claim 13 wherein theangular orientation of the layers are selected considering surfacequality of the workpiece along the first axis.
 18. An apparatus formaking a workpiece, comprising: a vat configured to contain a liquidresin; a platform movable along a build axis within the vat; a projectoroperable to project a patterned image of radiant energy comprising rowsand columns of pixels; and means for changing a relative angularorientation of the platform and the projector about the build axis. 19.The apparatus of claim 18 wherein the projector comprises: a radiantenergy source; and an image forming apparatus.
 20. The apparatus ofclaim 19 wherein the radiant energy source comprises an ultravioletflash lamp.